Hospital beds frequently employ electric devices to accomplish various tasks. The most frequent example of such devices involves the use of one or more electric motors to change the bed's configuration. Thus, the head portion, the knee portion, and the overall height of the bed may change to suit the patient's needs or desires. Additional devices may include lights, fans, or even call buttons for the nurses.
In most instances, the patient himself may operate the devices to suit his convenience. He does so by actuating various switches placed in his proximity. These switches normally remain near the patient in order that he may adjust his environment when the desire strikes him. Consequently, the patient may contact the switches even at a time when he does not intend to operate the device connected to them.
Thus, the likelihood for the patient to contact the switches in his proximity continually exists. More significantly, the patient may also make contact with the electrical current controlled by the switches. Expecially can this become a problem in light of the fact that metal components of the bed, light stands, and other furniture continually surround the patient. Furthermore, liquids of almost every description remain ubiquitously near the patient and may spill upon him during his normal activities. These liquids can reduce the normal skin electrical resistance and also provide a path for current to actually reach the portion of the switches contacted by the patient. Thus, the usual activities of the patient in bed may easily result in electric current from the connections to the switches contacting his skin. Moreover, the skin may have contemporaneously suffered reduced resistance and protection because of the liquids in his proximity. Should a significant potential or current thus pass to the patient, he could suffer a serious or even fatal electrical shock. This becomes a particular danger since the patient may have suffered reduced strength and concomittantly increased susceptibility due to his illness.
All manufacturers of electrical devices actuatable by a patient must take cognizance of this potential for shock. They must, therefore, devise a scheme that avoids high voltage, current or electrical energy levels in close proximity to the switches that a patient may contact. Accordingly, hospital bed manufacturers have incorporated various systems for keeping the high voltage, current or electrical energy levels necessary to operate the bed away from the patient.
One manufacturer has employed a pneumatic system which provides a barrier of air between the patient's switches and the electric current. When a patient actuates a switch, air pressure flows along a conduit to turn on or off a piece of electrical equipment, such as a motor.
The pneumatic system, however, suffers from considerable shortcomings that limit its actual commercial utility. First, this type of hospital bed requires all the components for a complete pneumatic system. Thus, a motor for the air pump as well as fluid-tight conduits become essential items, drastically increasing the bed's cost. Furthermore, the conduits can degenerate with time and lose their fluid-tightness. Moreover, sharply bending the conduits can completely and inadvertently close them off, rendering the system useless.
A. P. Petzon et al., in their U.S. Pat. No. 3,716,876, show a hospital bed employing lights and photo cells to separate the high voltage of the electrical motor from the low voltage used in the switches contacted by the patient. Again, however, the utilization of the light system requires the addition of expensive components, as well as introducing reliability and maintenance problems. Moreover, a foreign object becoming lodged between the lights and the photo cells can likely render the system totally inoperative.
Moreover, the light bulb and photo cells have a very slow response time. Stated otherwise, the bulb's performance displays a hysterisis effect. This lag becomes particularly troublesome when a person operating a motor in one direction rapidly fluctuates the switch to move it in the opposite direction. This may happen, for example, if the bed section moved beyond the desired point. With the slow response time of the light bulbs, especially in turning off, this attempted rapid reversal of the motor may actually result in power passing simultaneously through both the motor's forward and reverse windings. Current simultaneously flowing through both windings of a large motor can possibly damage it. It may also destroy the solid-state switches controlling the motor's current.
The use of reed relays to separate high voltage from the patient's controls appears in, inter alia, U.S. Pat. No. 3,913,153 to J. S. Adams et al. While the bed shown in their patent has worked satisfactorily in the field, it has only the small amount of insulation in the reed relay itself to isolate the circuitry in patient's controls from the high voltages used to power the motors. Moreover, reed relays represent expensive items to add to the circuit.
Additionally, reed relays present a reliability problem. The contacts have on occasion stuck together and remained closed, even after the discontinuance of the current, which should cause them to open. This continued closure of the contacts maintains the current flow to the particular motor involved. Upon the actuation of the switch to operate the motor in the reverse direction, that motor will again receive current in both its forward and reverse windings. As discussed above, the current in both windings of the motor may possibly affect it deleteriously. More importantly, these circuits with reed relays use them to operate triacs which control the current received by the motors. Supplying current through both of the triacs controlling the forward and reverse windings of a motor can damage and likely destroy them.
Furthermore, the reed relays do not represent a power-transferring device. The energy from the coil half of the reed relay simply cannot pass over to its coupled triac to turn on the motor. Accordingly, the circuit requires the expense of a further power supply to energize the triac coupled to the reed relay.
Electric motors in other circumstances, have also submitted to electronic control. J. Futamura, in his U.S. Pat. No. 3,686,557, switches a reversible motor on in either of its directions through the use of triacs coupled through transformers to oscillators. The circuit allows the appropriate oscillator to keep its coupled triac conducting sufficiently long for the motor to effect a needed change in a piece of controlled apparatus. However, no need exists for Futamura to guard his circuit from the voltages operating the motor and, consequently, he does not attempt this task.
In his U.S. Pat. No. 3,898,553, U. M. Van Bogget provides a circuit which periodically switches the current to a load on and off. He uses a triac as the specific switching element. To maintain the triac in the conducting state, he provides it with oscillator pulses having a frequency approximately twenty to forty times as great as that of the alternating current received by the load. The high pulse rate assures that the triac turns on promptly when desired. It need not wait an appreciable portion of a cycle of the alternating current supplied to the load.
Van Bogget, however, simply relates to the establishment of time intervals for the load to remain on or off. He has no concern for protecting any part of the circuit from the high voltages operating the load. Nor does he relate at all to the manually controlled switching devices as used in hospital beds. More specifically, he also provides no protection for such manual switches from the high voltages operating the load. Consequently, he proves of no benefit for hospital beds.
H. Wakamatsu et al. 's U.S. Pat. No. 3,986,093 employs a motor to passively wrap an automotive seat belt around a passenger in an automobile. They control the motor through the use of CMOS circuits which receive, as inputs, the positions of various switches connected to the door, the seat, and like. A resistance-capacitance (RC) segment coupled to the input of the CMOS components provides a slight delay to reduce the "chattering" in the circuits for the door seat switch. Alan R. Miller, in his article "Adaptive Motor Starter Delays When Necessary" in Electronics of July 24, 1975, produces an RC delay with CMOS components controlling the motor of an air conditioner. In IBM Technical Disclosure Bulletin 13, 519 (1970), D. J. Kostuch, in his article "Time Delay for Mosfet Integrated Logic", provides a delay in the excitation of the output of a Mosfet integrated circuit. However, the Mosfet circuit's output returns to its normal state promptly after the disappearance of the exciting signal. Again, none of these control circuits discuss preventing the voltage utilized by a load from reaching switches contacted by individuals.
The G. E. SCR Manual, 5th edition, 1972, on pages 115-116, 348-349, and 265, and the Guidebook of Electronic Circuits by John Marcus, 1974, discuss the use of SCRs, triacs, and thyristers to control an a.c. current passing to various loads including motors and light bulbs. The circuits employ pulse transformers to electrically isolate one section of the circuit from another. They do not, however, protect one portion of the circuit having very low voltages from a separate portion operating at high voltages. Accordingly, they do not satisfy the needs of hospital beds which can allow only very limited amounts of current to pass to the switches contacted by a patient.